Nowadays, the Micro-Electro-Mechanical System (MEMS) is a newly developed field that every country in the world is actively interfering and participating in. MEMS is a miniature system which generates a predict movement. With the advance of the modem technology, the system size and volume have been smaller and more delicate. The system miniaturization has led to many advantages. (1) Mass production: Several hundreds to several thousands of mechanical components can be formed on a single one silicon chip at the same time. Similar to the IC manufacture, the manufacture cost can be reduced accordingly. (2) Miniaturization: A quite small but still having high accuracy mechanical component can be produced by applying the method for optical image. (3) Preciseness: The manufacturing process of the micro-mechanical technology is quite accurate. (4) Integration: When manufacturing the mechanical elements, the electronic elements can be made at the same time. In other words, the mechanical elements and the electronic elements can be integrated on the same silicon chip. Particularly, the Micro-Electro-Mechanical System mainly utilizes the systematic technology, the micro-technology, and the material and effect technology for manufacturing the micro-detector, signal processor, micro-actuator, and etc. The future application fields are very wide, including manufacture, automation, information and communication, aerospace industry, transportation, civil construction, environmental protection, agriculture and fishery, and medical science.
In the past, the conventional micro-detector or micro-actuator only has in-plane motion, that is, a single-direction movement or a movement parallel to the chip surface. Recently, the micro-actuator having the feature of being movable in 2-dimensioal direction has been largely studied. Therefore, the out-of-plane motion, the movement direction unparallel to the chip surface, has been applied to the industry gradually. The out-of-plane motion including side movement and lateral movement, in fact, plays an important and key role in the optical application, such as the optical switch or the photo scanning. With the respect of the actuation driven by static electricity, the out-of-plane motion can be achieved by the in-plane actuator, horizontal comb actuator and vertical comb actuator. As shown in FIG. 1, FIG. 1(a) is an in-plane actuator, FIG. 1(b) is a horizontal comb actuator and FIG. 1(c) is a vertical comb actuator. Among these actuators, only the vertical comb actuator provides a wider movement and a direct moving mechanism. Please refer to FIG. 1(c). A vertical comb actuator is usually a relative comb structure, which includes an upper electrode 82 as the movable electrode and a lower electrode 80 as the fixed electrode. Since the upper electrode 82 and the lower electrode 80 are designed to have a height difference therebetween, a vertical actuating force is generated by the electric field having unsymmetrical directions.
Presently, the body structure made of the single crystal silicon (SCS) is very common in MEMS. Although the single crystal silicon is a brittle material, the flexibility of the structure is significantly increased owing to the particular feature of the MEMS structure that the volume is extremely small. Other than that, the mechanical strength of the MEMS structure is higher than that of most metals and alloys. The MEMS structure not only has no problem of the thin film stress, but also has a smaller signal shift induced by temperature due the fact that it has a thermal expansion coefficient smaller than that of a metal. Besides, the method using the SCS material for manufacturing the high-aspect-ratio-microstructure (HARM) has been transformed from the traditional bulk micromachining which has the shape limitation into an advanced method that is able to manufacture various thick structures. In addition, the manufactured thick structure owns many advantages including a higher driving frequency, a smaller dynamic distortion of the structure, a smaller effect from the out-of-plane perturbation motion, and a larger range of the structural rigidity.
However, the manufacturing method for a high-aspect-ratio-microstructure (HARM) having a height difference is still heavy and complicated. Please refer to FIGS. 2(a)-(n) which show the steps of the conventional manufacturing method for a high-aspect-ratio-microstructure (HARM). FIGS. 2(a)-(f) show the manufacturing steps for the upper electrode of the HARM. FIGS. 2(g)-(m) show the manufacturing steps for the lower electrode of the HARM. First, a silicon substrate 11 is provided. An etching process is performed from the back by using the photoresist 12 as a mask, as shown in FIG. 2(a). After the photoresist 12 is removed, the aurum/chromium layer 13 is coated on the silicon substrate 11, as shown in FIG. 2(b). Then, by using the photoresist 14 as a mask, the aurum/stannum layer 15 is grown on a part of the aurum/chromium layer 13 by electroplating, as shown in FIG. 2(c). Then, the photoresist 16 is covered on the aurum/stannum layer 15 as a mask, and the photoresist 14 and partial aurum/chromium layer 13 are removed, as shown in FIG. 2(d). Next, the comb-shaped patterning mask 17 is formed on the silicon substrate 11, as shown in FIG. 2(e). The inductively coupled plasma-reactive ion etching (ICP-RIE) is performed so as to define the thickness of the comb-shaped upper electrode, as shown in FIG. 2(f). Therefore, the comb-shaped upper electrode 18 is accomplished so far. The lower electrode is then manufactured as follows. First, the silicon substrate 21 is provided. An etching process is performed from the back by using the photoresist 22 as a mask, as shown in FIG. 2(g). Then, the silicon substrate 21 is fixed on the glass substrate 23 through anode bonding, as shown in FIG. 2(h). The lateral sides of the silicon substrate 21 are etched by using the photoresist 24 as a mask, as shown in FIG. 2(i). After the photoresist 24 is removed, the aurum/chromium layer 25 is coated on the silicon substrate 21, as shown in FIG. 2(j). Next, the photoresist 26 is covered on the aurum/chromium layer 25 formerly coated on the lateral sides of the silicon substrate 21. A part of the aurum/chromium layer 25 and the partial silicon substrate 21 are then removed, as shown in FIG. 2(k). Next, the comb-shaped patterning mask 27 is formed on the silicon substrate 21, as shown in FIG. 2(l). The inductively coupled plasma-reactive ion etching (ICP-RIE) is performed so as to define the thickness of the comb-shaped lower electrode, as shown in FIG. 2(m). Therefore, the comb-shaped lower electrode 28 is accomplished accordingly. Lastly, the accomplished upper electrode 18 is reversed and aligned with the lower electrode 28 for forming a chip by fixedly connecting the upper electrode 18 and the lower electrode 28, as shown in FIG. 2(n). Accordingly, a high-aspect-ratio-microstructure (HARM) having a height difference is accomplished.
From the above description, it is known that the manufacturing method for the HARM having a height difference is still very complex. When a manufacturing process has too many steps, the quality and the stability of each step are hard to control, which might result in an unstable manufacturing process. On the other hand, since the upper electrode 18 and the lower electrode 28 are manufactured separately, an accurate connecting process is absolutely required in order to solve the alignment problem therebetween. Since the accuracy of the internal distance between the upper electrode 18 and the lower electrode 28 will seriously affect the actuation stability, the requirement for the alignment accuracy is very high. If the alignment between the upper electrode 18 and the lower electrode 28 is not accurate enough, both the electrodes can not move smoothly in the electric field for generating a stable actuation force. Therefore, the allowable alignment error is quite small.
Besides, in order to help the alignment process during the connecting step, enough interval distance between the two electrodes must be remained. That is, the interval distance is limited within a certain range when manufacturing the upper electrode and the lower electrode. In fact, a lengthier interval distance would reduce the generated actuation force, and consequently the actuator effect would be affected.
In addition, the above manufacturing process for the silicon chip often has the unavoidable problem of not having enough conductive depth for the structure. Since the doped depth of silicon is mostly limited within 10 micrometer, the deeper part of the thick structure is usually not conductive, which affects the actuation and the detection feature. Presently, the conductivity is achieved by additionally attaching a conductive/dielectric layer to the structure. However, this solution still generates other problems in the latter manufacturing process, such as the difficulty for chip connection, the hardship to attach a metal lateral wall on the structure, and the machining limitation. The advantages from a pure silicon structure would disappear as well.
On the other hand, since the generated actuation force is affected by the motion of the upper electrode, the design for the elastic spring (deformation) which is served as the distortion axis is very critical. If the structure of the elastic spring (deformation) is too thick, the allowable range of the rotational angle will be decreased. However, the accomplished structure obtained from the above manufacturing process can only have one identical thickness. The thickness of the body structure can not be maintained if the thickness of the distortion axis is trimmed or decreased. Therefore, the conventional manufacturing process has a certain limitation for the thickness of the elastic spring (deformation).
From the above description, it is known that how to simplify the complex manufacturing process for the high-aspect-ratio-microstructure (HARM) and solve the alignment problem between the upper and lower electrodes in order to develop a HARM manufacturing process with higher stability, fewer design limitation and better manufacturing ability have become a major problem waited to be solved in the industry. In order to overcome the drawbacks in the prior art, a selective etching method with lateral protection function, which is applied to the manufacture of the 3-D components, is provided in the present invention.